in reply to (tye)Re: When to use Prototypes?
in thread When to use Prototypes?

Now I only use prototypes for making compile-time constants and to emulate map's and grep's ability to take bare blocks as code references (and this latter use is quite rare and really only gives a very minor benefit of not having to see "sub" in front of each block).

Yeah the bare block trick was what I meant by 'unusual situations' :-) A quick question (which I could answer myself after a bit of testing, but it seems like a point that should be shared) doesn't it also coerce a statement into a block as well?

My second question is how do prototypes help you with compile time constants? I assume you are talking about something like 

sub LOW_PRE_PI {3.141}

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and I dont see where prototypes come into play. Am I missing something?

Yves / DeMerphq
--
Have you registered your Name Space?

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Re: Re: (tye)Re: When to use Prototypes?
by danger (Priest) on Nov 09, 2001 at 23:15 UTC

    When you use an empty prototype (no args) for constants, the parser then knows not to interpret anything that follows as arguments for to that sub: 

    sub LOW_PRE_PI {3.141}
sub HI_POST_PI () {3.142}

my @n_proto = (1, LOW_PRE_PI + 5, 11, 15);
my @y_proto = (1, HI_POST_PI + 5, 11, 15);

print "@n_proto\n@y_proto";
__END__
Output:
1 3.141
1 8.142 11 15

    [download]

    In the first case, everything following the unprototyped version is taken as an argument list for the subroutine.

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      Yes, that is one difference, but I find code that makes use of that difference to be a bit of a problem. I'd probably write HI_POST_PI()+5 if I noticed.

      The difference that I'm talking about is that subroutines with a () prototype that also do nothing but return a value become compile-time constants. Perl can replace instances of calls to these with the constant returned at compile time, allowing lots of interesting tricks: 

      > Perl -MO=Deparse -w
use strict;
sub DEBUG() { 0 }
sub SymRef() { "My::Package::hash" }
sub PI() { 3.141592 }
if(  DEBUG  ) {
    warn "Okay, we are debugging...\n";
    do_lots_of_debugging_stuff();
}
$My::Package::hash{key}= "value";
print SymRef->{key},$/;
print PI()+5,$/;
__END__
sub DEBUG () {
    0;
}
sub SymRef () {
    'My::Package::hash';
}
sub PI () {
    3.141592;
}
'???';
$My::Package::hash{'key'} = 'value';
print $My::Package::hash{'key'}, $/;
print 8.141592, $/;
- syntax OK

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      Without the () prototypes we'd get: 
      sub DEBUG {
    0;
}
sub SymRef {
    'My::Package::hash';
}
sub PI {
    3.141592;
}
if (DEBUG ) {
    warn "Okay, we are debugging...\n";
    do_lots_of_debugging_stuff ;
}
$My::Package::hash{'key'} = 'value';
print SymRef()->{'key'}, $/;
print PI() + 5, $/;
- syntax OK

      [download]
      and, if we ran the code, we'd get: 
      Can't use string ("My::Package::hash") as a HASH ref
while "strict refs" in use

      [download]

              - tye (but my friends call me "Tye")
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Re: Re: (tye)Re: When to use Prototypes?
by blakem (Monsignor) on Nov 10, 2001 at 01:21 UTC
    Yesterday lestrrat showed how use constant $DEBUG works its magic Although he didn't specifically point out the bare prototype, it is there in the Deparsed output...

    -Blake

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